Download math hands

math
hands
Trigonometry
07 notes
Famous IDs: Sum-Angle Identities
Main Idea
We continue to expand the list of very famous trigonometric identities, and to practice our proving skills. We
now prove the second most famous/most important trigonometric identity. By tweaking this identity, we will
derive many, many more famous and important trigonometric identities. In some sense, this identity, cos(a − b) =
cos a cos b + sin a sin b, is the Mother Of Them All. We are now ready to learn it as well as all the famous variations
listed below.
Sum-Angle Identities
the M.O.T.A. 7−→
cos(a − b) = cos a cos b + sin a sin b
cos(a + b) = cos a cos b − sin a sin b
sin(a + b) = sin a cos b + cos a sin b
sin(a − b) = sin a cos b − cos a sin b
tan a + tan b
1 − tan a tan b
tan a − tan b
tan(a − b) =
1 + tan a tan b
tan(a + b) =
EXAMPLE 1 Mother Of Them All (by starting with something amazingly creative) Prove the following
identity
cos(a − b) = cos a cos b + sin a sin b
Solution: In order to prove this one, we need several key ingredients. First, we need to be well versed in finding
distances between two points. That is, we need to know that the distance squared, d2 , between two points, (A, B)
and (C, K) is given by d2 = (A − C)2 + (B − K)2 . Next, we need to be well versed in finding the x and the y
coordinates for a reference triangle where the hypothenuse is r and the angle is θ. Finally, we need an amazingly
creative idea, apparently unrelated at first, but obviously divinely inspired once it delivers the goods.
preliminaries: review distance
(4, 6)
6
4
d
2
4
6
2
b
8 10 12 14
-14 -12 -10 -8 -6 -4 -2
-2
-4
distance by pyth.
-4
-6
d2 = (8−4)2 +(6−2)2
-6
c
2007-2009
MathHands.com
b
4
(8, 2)
8−4
(?, ?)
6
b
6−2
2
-14 -12 -10 -8 -6 -4 -2
-2
review coordinates
math
hands
(r cos θ, r sin θ)
r
y
x
θ
2
4
cos θ =
6
x
r
8 10 12 14
sin θ = yr , thus
x = r cos θ and y = r sin θ
pg. 1
math
hands
Trigonometry
07 notes
the brilliant idea: hold this triangle up, then walk away...
the brilliant idea: hold this triangle up, then walk away...
Now, the idea is to calculate the distance, d, in both cases, before the triangle falls and after it has fallen.
(cos a, sin a)
d
1
1
a
(cos(a − b), sin(a − b))
(cos b, sin b)
d
a−b
b
(1, 0)
d2 = [cos(a − b) − 1]2 + [sin(a − b) − 0]2
(from dist. formula)
d2 = (cos a − cos b)2 + (sin a − sin b)2 (from dist. formula)
2
2
2
2
2
d = cos a − 2 cos a cos b + cos b + sin a − 2 sin a sin b + sin b
(FOIL, alg)
d2 = cos2 a + sin2 a + cos2 b + sin2 b − 2 cos a cos b − 2 sin a sin b
(to use Pythagoras ID, alg)
d2 = 1 + 1 − 2 cos a cos b − 2 sin a sin b
(use Pythagoras ID, alg)
d2 = 2 − 2 cos a cos b − 2 sin a sin b
d2 = cos2 (a − b) − 2 cos(a − b) + 1 + sin2 (a − b)
(FOIL, alg)
d2 = cos2 (a − b) + sin2 (a − b) − 2 cos(a − b) + 1
(to use Pythagoras ID, alg)
d2 = 1 − 2 cos(a − b) + 1
(use Pythagoras ID, alg)
d2 = 2 − 2 cos(a − b)
(alg)
(alg)
Now, we conclude the square of the distance, d2 is the same before and after the triangle has fallen. Thus,...
d2 = d2
2 − 2 cos a cos b − 2 sin a sin b = 2 − 2 cos(b − a)
−2 cos a cos b − 2 sin a sin b = −2 cos(b − a)
cos a cos b + sin a sin b = cos(b − a)
(from above)
(from work above)
(algebra)
(algebra)
yip-kaei-yeah!!
c
2007-2009
MathHands.com
math
hands
pg. 2
math
hands
Trigonometry
07 notes
EXAMPLE 2 Famous Sum-Angle Id. (by tweaking a known Id.) Prove the following identity
cos(x + y) = cos x cos y − sin x sin y
Solution: We begin with the MOTA Identity, which we have just proven. Since it is true for all angles a and b, we can apply
it for a = x and b = −y. After these values are substituted, we clean it up and voila!
cos(a − b)
=
cos a cos b + sin a sin b
cos[x − (−y)]
=
cos x cos(−y) + sin x sin(−y)
Known Id, MOTA
cos(x + y)
=
cos x cos y + sin x sin(−y)
cos(x + y)
=
cos x cos y + (sin x)(− sin y)
cos(x + y)
=
cos x cos y − sin x sin y
the Tweak, substitute a = x and b = −y
clean up, cosine is an even function, see Famous Ids
clean up, sine is odd function, see Famous Ids
clean up, QED
EXAMPLE 3 Sum-Angle for Sine (by tweaking a known Id.) Prove the following identity
sin(a − b) = sin a cos b − cos a sin b
Solution: we will tweak a known identity. Recall also the Co-Function identities. The main difference between a sine and a
cosine is the ’co’ for co-mplimentary angles, thus replacing any angle θ with 90◦ − θ results in the co-function ratio. With
this in mind we will substitute x = (90◦ − a) and y = b
cos(x + y)
=
cos x cos y − sin x sin y
cos[(90◦ − a) + b]
=
cos(90◦ − a) cos b − sin(90◦ − a) sin b
◦
◦
known ID
◦
the tweak, sub
cos[90 − (a − b)]
=
cos(90 − a) cos b − sin(90 − a) sin b
sin[(a − b)]
=
sin a cos b − cos a sin b
co-function Ids
sin(a − b)
=
sin a cos b − cos a sin b
goods delivered! famous Id!
c
2007-2009
MathHands.com
math
hands
algebra
pg. 3
math
hands
Trigonometry
07 exercises
Famous IDs: Sum-Angle Identities
1. Prove and OWN everyone of these famous identities.
Sum-Angle Identities
the M.O.T.A. 7−→
cos(a − b) = cos a cos b + sin a sin b
cos(a + b) = cos a cos b − sin a sin b
sin(a + b) = sin a cos b + cos a sin b
sin(a − b) = sin a cos b − cos a sin b
tan a + tan b
tan(a + b) =
1 − tan a tan b
tan a − tan b
tan(a − b) =
1 + tan a tan b
2. Prove the following non-famous identity.
cos(4x) = cos x cos 3x − sin x sin 3x
3. Prove the following non-famous identity.
cos(3x) = cos x cos 2x − sin x sin 2x
4. Prove the following non-famous identity.
cos(2x) = cos x cos x − sin x sin x
5. Prove the following non-famous identity.
cos(4x) = cos 2x cos 2x − sin 2x sin 2x
6. Prove the following non-famous identity.
cos(5x) = cos 2x cos 3x − sin 2x sin 3x
7. Prove the following non-famous identity.
tan(10x) =
tan 12x − tan 2x
1 + tan 12x tan 2x
tan(10x) =
tan 8x + tan 2x
1 − tan 8x tan 2x
8. Prove the following non-famous identity.
c
2007-2009
MathHands.com
math
hands
pg. 4
math
hands
Trigonometry
07 exercises
9. Prove the following non-famous identity.
cos(8x) = cos 10x cos 2x − sin 10x sin 2x
10. Prove the following non-famous identity.
cos(8x) = cos 4x cos 4x − sin 4x sin 4x
11. Prove the following non-famous identity.
cos 10x cos 2x − sin 10x sin 2x = cos 4x cos 4x − sin 4x sin 4x
12. Without calculators determine if the following is true, then explain...
√
√
6+ 2
◦
cos 15 =
4
13. Without calculators determine if the following is true, then explain...
s
√
√
√
6+ 2
2+ 3
=
4
4
14. Prove and OWN everyone of these famous identities.
Double-Angle Identities
cos(2a) = cos2 a − sin2 a
cos(2a) = 2 cos2 a − 1
cos(2a) = 1 − 2 sin2 a
2 tan a
tan(2a) =
1 − tan2 a
sin(2a) = 2 sin a cos a
1 − cos 2θ
sin2 θ =
2
cos2 θ =
1 + cos 2θ
2
15. Prove and OWN everyone of these famous identities.
Product-to-Sums Identities
1
[cos(a − b) − cos(a + b)]
2
1
cos a cos b = [cos(a − b) + cos(a + b)]
2
1
sin a cos b = [sin(a + b) + sin(a − b)]
2
sin a sin b =
16. Prove and OWN everyone of these famous identities.
c
2007-2009
MathHands.com
math
hands
pg. 5
math
hands
Trigonometry
07 exercises
Sums-to-Products Identities
a−b
a+b
cos
sin a + sin b = 2 sin
2
2
a−b
a+b
sin a − sin b = 2 sin
cos
2
2
a−b
a+b
cos
cos a + cos b = 2 cos
2
2
a+b
a−b
cos a − cos b = −2 sin
sin
2
2
17. (**)Prove the identity
sin(2x) = 2 sin x cos x
c
2007-2009
MathHands.com
math
hands
pg. 6